The present invention relates to an optically activated switch for high power RF signals. More specifically, but without limitation thereto, the present invention relates to photo-voltaic switches having relatively low optical activation power.
Optically-activated RF switches include photoconductive switches, phototransistors, surface-depleted optical FET's, and photovoltaic field effect transistors. A photoconductive switch typically has an on-resistance that is linearly proportional to the power of the optical control signal, thus a low on-resistance of about 2 ohms typically requires an optical control signal power of over 50 mW. Switching time is generally linearly proportional to the carrier lifetime of the photoconductor and inversely proportional to the on-resistance. A high speed photoconductive switch thus tends to have the disadvantage of a high on-resistance. Phototransistors generally require high optical control power for a low on-resistance, and consequently suffer from the problem of a low off-resistance. The load resistance of a typical RF application, however, such as a dipole antenna, is approximately 73 ohms. An RF switch capable of switching a 100 W signal for such applications should have an off-resistance of at least 10k ohms to adequately isolate the input from the output in the off state and an om-resistance of about 1 ohm.
U.S. Pat. No. 4,825,081 issued on Apr. 25, 1989 to Wille et al discloses series connected PIN diodes with P electrodes connected to N electrodes, i.e. commonly aligned polarity (col. 2, ln. 63-66) as a switching element controlled by a light source, but suffers the disadvantage of requiring a reverse bias voltage source (col. 3, ln. 1-7).
A need therefore exists for an optically activated RF switch having a low on-resistance, a high off-resistance, and low optical control power.